The present invention relates to organic electroluminescent devices which comprise triazine derivatives as electron-transport materials.
The structure of organic electroluminescent devices (OLEDs) in which organic semiconductors are employed as functional materials is described, for example, in U.S. Pat. Nos. 4,539,507, 5,151,629, EP 0676461 and WO 98/27136. However, further improvements are still desirable. Thus, there is still a need for improvement, in particular, with respect to the lifetime, the efficiency and the operating voltage of organic electroluminescent devices. It is furthermore necessary for the compounds to have high thermal stability and a high glass-transition temperature and to be sublimable without decomposition.
Improvements in the properties are still desirable, in particular, in the case of the electron-transport materials since it is precisely also the properties of the electron-transport material that have an essential influence on the above-mentioned properties of the organic electroluminescent device. In particular, there is a need for improvement in electron-transport materials which simultaneously result in good efficiency, a long lifetime and a low operating voltage. In particular, the properties of the electron-transport material are also frequently limiting for the lifetime, the efficiency and the operating voltage of the organic electroluminescent device.
It would be desirable here to have available electron-transport materials which result in better electron injection into the emitting layer, since an electron-richer emission layer results in better efficiency. In addition, better injection can reduce the operating voltage. Further improvements in the electron-transport material are therefore necessary for this purpose.
There is furthermore generally still a need for improvement in the processability of the materials, since many materials used in organic electro-luminescent devices in accordance with the prior art tend to crystallise on the vapour-deposition source in the process for the production of the electroluminescent device and thus block the vapour-deposition source. These materials can therefore only be employed with increased technical complexity in mass production.
Electroluminescent devices which use AlQ3 as electron conductor have already been known for some time and were already described in 1993 in U.S. Pat. No. 4,539,507. AlQ3 has since then frequently been used as electron-transport material, but has a number of disadvantages: it cannot be vapour-deposited without leaving a residue, since it partially decomposes at the sublimation temperature, which represents a major problem, in particular, for production plants. This has the consequence that the vapour-deposition sources have to be continually cleaned or exchanged. Furthermore, AlQ3 decomposition products enter the OLED, where they contribute to a shortened lifetime and reduced quantum and power efficiency. In addition, AlQ3 has low electron mobility, which results in higher voltages and thus in lower power efficiency. In order to avoid short circuits in the display, it would be desirable to increase the layer thickness; this is not possible with AlQ3 owing to the low charge-carrier mobility and the resultant increase in voltage. The charge-carrier mobility of other electron conductors (U.S. Pat. No. 4,539,507) is likewise too low to build up thicker layers therewith, where the lifetime of the OLED is even worse than on use of AlQ3. The inherent colour (yellow in the solid) of AlQ3, which can result in colour shifts due to reabsorption and weak re-emission, especially in the case of blue OLEDs, also proves unfavourable. It is only possible to produce blue OLEDs here with considerable losses in efficiency and colour location.
Thus, there continues to be a demand for electron-transport materials which result in good efficiencies and simultaneously in long lifetimes in organic electroluminescent devices. Surprisingly, it has now been found that organic electroluminescent devices which comprise certain triazine derivatives—indicated below—as electron-transport materials have significant improvements over the prior art. By means of these materials, it is possible simultaneously to obtain high efficiencies and long lifetimes, which is not possible using materials in accordance with the prior art. In addition, it has been found that, in addition, the operating voltages can be significantly reduced, which results in higher power efficiencies.
It has furthermore been found that organic electronic devices which comprise triazine derivatives as electron-transport materials in combination with an organic alkali-metal compound have significant improvements over the prior art. Using this material combination, high efficiencies and long lifetimes are achieved simultaneously, and the operating voltages are reduced.
U.S. Pat. Nos. 6,229,012 and 6,225,467 disclose the use of fluorene and biphenyl derivatives which are substituted by triazine groups as electron-transport material in OLEDs. However, the patents do not reveal that, in particular, materials which contain a spirobifluorene group instead of the fluorene group offer significant advantages over the materials disclosed in the above-mentioned patents.
WO 05/053055 discloses the use of triazine derivatives, in particular spirobifluorene derivatives which are substituted by triazine groups, as hole-blocking material in a hole-blocking layer in phosphorescent OLEDs. However, the application does not reveal that these materials are also suitable as electron-transport materials for fluorescent electroluminescent devices.